The present invention relates to a ceramic filter used for water purification or the like and a method for purifying water using the same.
Ceramic filters have a separation layer, which controls the pore size of the filter, formed on a surface of a substrate formed from a ceramic porous body or a surface of an intermediate layer formed on the substrate. Since such ceramic filters have a high physical strength, endurance, corrosion resistance and the like, they are used for removing suspended matters, bacteria or dust particles from liquid or gas in a wide variety of fields including water treatment, exhaust gas treatment, and medical, pharmacy or food industry field.
A known ceramic filter is formed by bonding alumina particles having a diameter of 12 μm on a support (substrate) made of cordierite with glass frits, bonding alumina particles having a diameter of 1.5 μm thereon with glass frits, firing the product at 1179° C., and to promote self-sintering of alumina particles having a diameter of 0.3 μm at 1179° C. (see U.S. Pat. No. 4,983,423 and U.S. Pat. No. 510,652).
U.S. Pat. No. 5,415,775 also discloses an embodiment of a filter having a support (substrate) and a separation layer formed thereon, the separation layer being made of titania and having a thickness of 25 μm and a pore size of 0.2 μm. However, when serving as a filter for water purification, for example, ceramic filters with an alumina separation layer are inferior to ceramic filters with a titania separation layer in fouling characteristic. To treat raw water with high turbidity, the raw water needs to be subjected to a pretreatment, such as coagulation, before filtration. However, even if fouling is reduced by any pretreatment, the filter with the separation layer of alumina still suffers significant fouling.
As for the filters with a titania separation layer, if the average pore size is below 0.08 μm, or if the film thickness is above 20 μm, a satisfactory fouling characteristic cannot be provided. In water purification, in particular, the filters still suffer significant fouling if dead-end filtration is adopted for the raw water with high turbidity. In addition, if the average pore size of the separation layer is above 1 μm, or if the thickness thereof is below 5 μm, the filter provides inadequate bacteria removal. Furthermore, if the separation layer formed on the intermediate layer is fired without adding any sintering aid to the aggregate particles, or if the intermediate layer is fired with a sintering aid other than glass frit added, the filter has a large maximum pore size and cannot provide high bacteria removal.
The present invention has been devised in view of such circumstances. An object thereof is to provide a ceramic filter having both good fouling characteristics and high bacteria removal capability, and that can be used suitably for water purification or the like, and a method for purifying water using the filter.